1. Field of the Invention
The invention relates in general to an isolation structure, and more particularly to an isolation structure applied to an non-volatile memory.
2. Description of the Related Art
Local Oxidation of Silicon (LOCOS) is the traditional isolation technique. At first a very thin silicon oxide layer 12 is grown on the wafer 10, the so-called pad oxide. Then a layer of silicon nitride 14 is deposited which is used as an oxide barrier. The pattern transfer is performed by photolithography. After lithography the pattern is etched into the nitride 14. The result is the nitride mask as shown in FIG. 1a, which defines the active areas for the oxidation process. The next step is the main part of the LOCOS process, the growth of the thermal oxide 12a (see FIG. 1b). After the oxidation process is finished, the last step is the removal of the nitride layer 14 (see FIG. 1c). The main drawback of this technique is the so-called bird's beak effect and the surface area which is lost to this encroachment.
The Shallow Trench Isolation (STI) is another isolation technique. The STI process starts in the same way as the LOCOS process. The first difference compared to LOCOS is that a shallow trench 18 is etched into the silicon substrate 10, as shown in FIG. 2a. A pattern of the resist 16 is transferred to the silicon nitride layer 14, the silicon oxide layer 12, and also part of the silicon substrate 10. After underetching of the oxide pad 12, also a thermal oxide 20 in the trench is grown in the Furnace, the so-called liner oxide (see FIG. 2c). But unlike with LOCOS, the thermal oxidation process is stopped after the formation of a thin oxide layer 20, and the rest of the trench 18 is filled with a deposited oxide 30, which is performed by chemical vapor deposition (see FIG. 2d). Next, the excessive (deposited) oxide is removed with chemical mechanical planarization (see FIG. 2e). At last the nitride mask 14 is also removed (see FIG. 2f). However, the price for saving space with STI is the larger number of different process steps.
Therefore, there exists a requirement of an isolation technique, being capable of solving the above problems without complicating the manufacturing process and increasing the cost.